H.264 not playing in Windows Media Player

OK, so I've exported a 720p video to H.264 format. The resultant .mp4 plays just fine on my Mac where it was created, in both QuickTime and the VLC Player. It also plays on another person's Mac (I'm assuming QuickTime) just fine.

But two people who have Windows machines cannot get the file to play in Windows Media Player. I don't think they've tried another player, but that's irrelevant because I need my output video to be able to play in WMP.

Is H.264 a poor choice in this case? That's always been my go-to format. I thought .mp4 was a universally playable format.

Thank you.